This study investigated whether the expression profile of GDF8 (myostatin), myogenic regulatory factors (MRFs: MYF5, MYOD1, MYOG (myogenin), and MYF6), and IGF-system (IGF1, IGF2, IGF1R) genes are correlated with anatomical muscle, nutrition level, and estimated breeding values (EBVs) for muscling, growth, and/or fatness. Real-time PCR was employed to quantitatively measure the mRNA levels of these genes in the semimembranosus (SM) and semitendinosus (ST) muscles of growing lambs. The lambs were sired by Poll Dorset rams with differing EBVs for growth, muscling, and fatness, and were fed either high or low quality and availability pasture from birth to ~8 months of age. With the exception of MYOD1, the mRNA levels of all genes examined in this study showed varying degrees of nutritional regulation. All the MRF mRNA levels were higher in the SM muscle than the ST muscle, whereas myostatin mRNA was higher in the ST muscle than the SM muscle. Interactions between muscle type and nutrition were detected for IGF2, MYF6, and myogenin, while positive correlations between IGF2 and IGF1R and between MYOD1 and myogenin mRNA levels were apparent in both muscles. At the genotypic level, subtle differences in mRNA levels suggested interactions between nutrition and sire EBV. The findings of this study confirm that the MRFs, IGFs, and myostatin genes are differentially affected by a variety of factors that include nutrition, muscle type, and sire EBVs. Together, these data suggest that this suite of genes has important roles during postnatal muscle growth, even at quite late stages of growth and development.